Mario Galaxy 1/2 are vital - you won't see a prettier Wii game than those.
Mario Party as well, that is a lot of fun, even if it can be frustrating.
Madworld is quite novel - if you see that going cheap then it's probably worth a shot, a strange game but one that I find myself playing more often than most Wii titles.
Monopoly. Just because it's more convenient than Monopoly, it's a pretty good version and allows custom rules.
Pikmin is good as well if you like a challenge. I know some people might dismiss it as some sort of kiddies game, it's not, it's bloody difficult and takes real strategy and determination to play properly. I found both Pikmin games very interesting, nostalgic too the way you find bits of broken NES controller and stuff.
Harvest Moon, Wonderful Life. That's on the GameCube, but it's the best farming simulator ever made, it's deeper and more entertaining than it's kiddie appearance lets on. Hell, I'm not even sure if I played that game the right way, but I played it for ages anyway - sitting fishing at night, tending crops and breeding animals, trying to remember to go to the shop. It's like a little microcosm where nothing much happens, but nothing much has to happen really, that game is more about finding out what sort of person you are than anything else. I don't play these sorts of games usually, I hate the 2D harvest moons, but this one is a work of art.
And Resident Evil 4 - now that is if you haven't already got it on another platform, it's another vital game, whether you play it on GC, PC, PS3, or whatever, everyone should play through RE4.
And also Lego Indiana Jones... not to play, but to 'allow' you to get the most from your Wii.
